In the past, percutaneous tube placements have been utilized to evacuate fluid collections or to form a passageway to insert a laparoscopic sheath. In all of these procedures, even though the trocar has three sharp edges, more than a moderate force is often needed to insert the trocar cannula. This poses a clear danger in that, with the sudden lack of resistance after penetration through resistant tissue, the instrument may be driven in too far and may cause serious damage to a viscus. In the case of the pleural cavity, one may injure the lung, heart or spleen and in the abdomen there is danger of injuring the intestine, aorta or other body structure. The trocar and cannula require the axial force necessary to cut through and dilate the tissues. In order to obtain the desired dilation and outward spread, the use of the conventional trocar and cannula results in a dangerous axial force or thrust.
Typical cannula and trocar assemblies are shown in such patents as Deniega U.S. Pat. No. 5,066,288, Holmes et al U.S. Pat. No. 4,931,042 and Knepshield et al U.S. Pat. No. 4,177,814. The Albertini U.S. Pat. No. 4,670,008 discloses a needle for injecting or removing liquid from a body comprising a trocar having a sharpened end disposed within a cannula with the external surface of the cannula having threads on its end to facilitate insertion. However, these devices do not provide an adequate solution to the problem of enabling the surgeon to determine precisely when the trocar has reached the desired body cavity so as to prevent damage to other body tissues.